The earthworking vehicles which are provided with lift arms and tilt links, like a loader, can perform various kinds of jobs according to the type of the implement attached thereto. For example, a loader which has a bucket attached as an implement can perform the jobs such as transferring loose products in harbor loading and unloading, reshuffling of yards, backfilling of piping ditches, or earth spreading. A loader which is equipped with a fork is useful for the transfer of palletized goods to and from warehouses, transfer of goods in the form of rolls or transferring and laying pipes in piping work. Therefore, a single earthworking vehicle can serve for multiple purposes by replaceably attaching thereto a bucket, fork, blade or other implements according to the nature of the work required.
As shown, for instance, in the specification and drawings of U.S. Pat. Nos. 3,760,883 issued Sept. 25, 1973, to B. D. Birk; 3,935,953 issued Feb. 3, 1976, to R. N. Stedman; and 4,116,347 issued Sept. 26, 1978, to T. Uchida; there have been known and in use various quick couplers which allow to attach a variety of implements quickly and detachably to an earthworking vehicle which is provided with lift arms and tilt links. However, as pointed out in detail in the specification of the above-mentioned U.S. Pat. No. 4,116,347, the known quick couplers invariably have drawbacks including complicated construction, high production cost, troubles which make the equipment inoperative within a short period of time, and inability of coupling the working implement securely and safely to the earthworking vehicle, and the like.
Furthermore, the specification and drawings of U.S. Pat. No. 4,116,347 issued Sept. 26, 1978, to T. Uchida discloses a quick coupler with a hitch which can be moved into stretched and folded states by a hydraulic operating means or a locking mechanism, similarly to the specification and drawings of the above-mentioned U.S. Pat. No. 4,116,347. This type of hitch is free from the above-mentioned drawbacks and capable of coupling working implements quickly and securely to the earthworking vehicle in an improved manner. However, there have still been problems for improving the hitch construction which is not yet simple enough due to the necessity for moving the whole hitch assembly into the stretched and folded states, incurring a production cost which is not sufficiently low enough.
Further, the specification and drawings of Japanese patent application No. 51-98784 disclose a quick coupler which is provided with a hitch assembly having an upper coupling element, a lower coupling element link means, and an intermediate coupling element consisting of a coupling block loosely fitted in the vicinity of a free end of the link means, and with a hook assembly having an upper receiving element, a lower receiving element and an intermediate receiving element consisting of a key-shaped element, securely preventing the hitch and hook assemblies from disengaging from each other in the forward and rearward directions by the engagement of the intermediate coupling and receiving elements. This type of quick coupler allows quick and secure attachment of a working implement to an earthworking vehicle suitably over a long period of time but still has a number of inherent problems in that its construction is not simple enough and its production cost is not low enough due to the necessity for the provision of the intermediate coupling elements on the hitch assembly in addition to the upper and lower coupling elements and the necessity for the provision of the intermediate receiving portion on the hook assembly in addition to the upper and lower receiving portions, and in that its operation is relatively complicated due to the necessity for rotating the intermediate coupling element of the hitch assembly by the tilt-back or tilt-forward operations while actuating other operating mechanisms in both the engaging and disengaging operations between the intermediate coupling element hitch of the hitch assembly and the intermediate receiving portion of the hook assembly, namely, in both the coupling and uncoupling operations of the hitch and hook.